When converting fossil fuel or biofuel to mechanical and/or thermal energy, the aim is to achieve high levels of efficiency. In this case, the conversion temperatures are often so high that during the conversion (combustion) nitrous oxides are formed to a significant extent.
As a release of the nitrous oxides is undesirable and hence to be avoided, the nitrous oxides are usually catalytically reduced. For elimination of the nitrous oxides the selective catalytic reduction method (also known as the SCR method) in particular has established itself as a technically easily controllable method that is used to remove nitrous oxides from exhaust gases of a combustion system, also an internal combustion engine, in particular a diesel engine.
In this method the nitrous oxides contained in the exhaust gas are converted in the presence of oxygen at a selective reduction catalytic converter by means of a suitable reducing agent into molecular nitrogen and water. As a reducing agent ammonia has won through against other reducing agents such as hydrocarbons or cyanuric acid.
Ammonia is a strong-smelling and—in a higher concentration—also toxic compound. The reducing agent needed to remove nitrous oxides from the exhaust gases of an internal combustion engine for example of a vehicle is therefore carried in a storage tank in the vehicle frequently in the form of an aqueous urea solution, from which the ammonia may be released, in particular by hydrolysis, in the quantity needed for the removal of nitrous oxides.
For this purpose, from DE 10 2004 054 238 A1 it is known for the aqueous urea solution to be added in a conveying direction from a storage tank via a delivery pump and a feed line through a delivery means to the exhaust gas of an internal combustion engine.